Cellular responses to changes in available oxygen (O2) levels are essential for normal development and physiology, and play a crucial role in disease. Hypoxia-inducible factors (HIFs) participate in these responses by stimulating the expression of genes involved in glycolysis, hematopoiesis, cell motility, and angiogenesis, all of which contribute to hypoxic adaptations. HIFs are heterodimeric transcription factors consisting of 1 (HIF-11 or HIF-21) and 2 (ARNT) subunits. Much attention has been focused on the ubiquitously expressed HIF-11 factor. However, the related protein HIF-21 clearly plays an important, as yet largely undetermined role in hypoxic adaptations. HIF-21 is highly expressed in endothelial cells (ECs) and regulates the expression of multiple genes that control angiogenesis such as VEGF, Tie2, and adrenomedullin. Moreover, HIF-21 preferentially regulates hypoxic VEGF expression in multiple cell types. Previous reports describe the effects of ablating HIF-11 in ECs, while little is known about HIF-21 function in these cells. To fully understand the complex role of hypoxia in controlling angiogenesis, particularly in response to pathological conditions such as tissue ischemia and solid tumors, it is important to establish the mechanisms by which HIF-21 functions in ECs and proangiogenic bone marrow-derived cells. This proposal will test the following hypothesis: HIF-21 plays critical and unique roles in regulating angiogenic responses to hypoxia, particularly during adult neovascularization. To test this hypothesis, we will (1) determine the role of HIF-21 in vascular EC function in vitro, (2) define the role of HIF-21 in ECs in vivo, and (3) determine the effects of HIF-21 ablation in the recruitment and function of proangiogenic bone marrow derived-monocytes (BMDMs). The proposed experiments are timely, in that recent reports suggest that HIF-21 (but not HIF-11) is critical to the progression of specific malignancies such as renal clear cell carcinoma, small cell lung carcinoma, neuroblastoma, and astrocytoma. The overall goal of these studies is to delineate how HIF-11 and HIF-21 differentially regulate hypoxic gene induction and how this contributes to angiogenesis, especially in adult pathophysiological settings. Project Narrative:Oxygen is essential for most life forms on earth. Therefore, responses to changes in oxygen availability are critical for development, physiology, and diseases such as the growth of solid tumors or tissue ischemia. Oxygen sensitive hypoxia-inducible factors participate in these responses and have been implicated in the growth of tumors such as renal clear cell carcinoma, small cell lung carcinoma, colon carcinoma, and neuroblastoma. This project will provide fundamental insights into how these factors regulate blood vessel growth in diseases and identify new therapeutic targets to treat cancer, stroke, and heart attack patients.